Accessing files in hadoop distributed cache

Question

I want to use the distributed cache to allow my mappers to access data. In main, I'm using the command

DistributedCache.addCacheFile(new URI("/user/peter/cacheFile/testCache1"), conf);

Where /user/peter/cacheFile/testCache1 is a file that exists in hdfs

Then, my setup function looks like this:

public void setup(Context context) throws IOException, InterruptedException{
    Configuration conf = context.getConfiguration();
    Path[] localFiles = DistributedCache.getLocalCacheFiles(conf);
    //etc
}

However, this localFiles array is always null.

I was initially running on a single-host cluster for testing, but I read that this will prevent the distributed cache from working. I tried with a pseudo-distributed, but that didn't work either

I'm using hadoop 1.0.3

thanks Peter

Answer 1

Problem here was that I was doing the following:

Configuration conf = new Configuration();
Job job = new Job(conf, "wordcount");
DistributedCache.addCacheFile(new URI("/user/peter/cacheFile/testCache1"), conf);

Since the Job constructor makes an internal copy of the conf instance, adding the cache file afterwards doesn't affect things. Instead, I should do this:

Configuration conf = new Configuration();
DistributedCache.addCacheFile(new URI("/user/peter/cacheFile/testCache1"), conf);
Job job = new Job(conf, "wordcount");

And now it works. Thanks to Harsh on hadoop user list for the help.